ALBERT

Description: To evaluate the accuracy of Fe 3+ and Fe 2+ ratios in silicate glasses determined by Mössbauer spectroscopy, we examine in detail the temperature (47–293 K) of Mössbauer spectra for two andesitic glasses, one quenched at 1 atm, 1400 °C (VF3) and the other at 3.5 GPa, 1600 °C (M544). Variable-temperature Mössbauer spectra of these two glasses are used to characterize the recoilless fraction, f , by two different methods—a relative method (RM) based on the temperature dependence of the ratios of Fe 3+ and Fe 2+ Mössbauer doublets and the second based on the temperature dependence of the center shift (CS) of the doublets. The ratio of the recoilless fractions for Fe 3+ and Fe 2+ , C T , can then be used to adjust the observed area of the Mössbauer doublets into the Fe 3+ /Fe ratio in the sample. We also evaluated the contributions of non-paramagnetic components to the Fe in the glasses by determining the influence of applied magnetic field on sample magnetization. Finally, for the VF3 glass, we determined the Fe 3+ /Fe independently by wet chemical determination of the FeO content combined with careful electron microprobe analyses of total Fe. Recoilless fractions determined with the CS method (CSM) are significantly smaller than those determined with the relative method and suggest larger corrections to room-temperature Fe 3+ /Fe ratios. However, the RM determinations are believed to be more accurate because they depend less on the assumption of the Debye harmonic model and because they produce more nearly temperature-independent estimates of Fe 3+ /Fe ratios. Non-linear responses of sample magnetizations to applied magnetic fields indicate that the glasses contain a small (0.4–1.1% for VF3) superparamagnetic component that is most likely to be nanophase precipitates of (Fe,Mg)Fe 2 O 4 oxide, but corrections for this component have negligible influence on the total Fe 3+ /Fe determined for the glass. For the VF3 glass, the Fe 3+ /Fe produced by uncorrected room-temperature Mössbauer spectroscopy [0.685 ± 0.014 in two standard deviation (2)] agrees within 3% of that determined by wet chemistry (0.666 ± 0.030 in 2). The Fe 3+ /Fe corrected for recoilless fraction contributions is 0.634 ± 0.078(2), which is 7.5% lower than the uncorrected room-temperature ratio, but also agrees within 5% of wet chemical ratio. At least for this andesitic glass, the room-temperature determination of Fe 3+ /Fe is accurate within analytical uncertainty, but room-temperature Mössbauer determinations of Fe 3+ /Fe are always systematically higher compared to recoilless-fraction corrected ratios.

Description: Density measurements on several hydrous (〈/=19 mole percent of H2O) silicate melts demonstrate that dissolved water has a partial molar volume (V&cjs1171;H2O) that is independent of the silicate melt composition, the total water concentration, and the speciation of water. The derived value for V&cjs1171;H2O is 22.9 +/- 0.6 cubic centimeters per mole at 1000 degrees C and 1 bar of pressure, whereas the partial molar thermal expansivity ( partial differentialV&cjs1171;H2O/ partial differentialT) and compressibility ( partial differentialV&cjs1171;H2O/ partial differentialP) are 9.5 +/- 0.8 x 10(-3) cubic centimeters per mole per kelvin and -3.2 +/- 0.6 x 10(-4) cubic centimeters per mole per bar, respectively. The effect of 1 weight percent dissolved H2O on the density of a basaltic melt is equivalent to increasing the temperature of the melt by approximately 400 degrees C or decreasing the pressure of the melt by approximately 500 megapascals. These measurements are used to illustrate the viability of plagioclase sinking in iron-rich basaltic liquids and the dominance of compositional convection in hydrous magma chambers.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Ochs 3rd -- Lange -- New York, N.Y. -- Science. 1999 Feb 26;283(5406):1314-1317.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109-1063, USA.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/10037599" target="_blank"〉PubMed〈/a〉

Description: Three crystal-poor obsidian samples (one dacite, 67 wt % SiO 2 ; two rhyolites, 73 and 75 wt % SiO 2 ), which erupted effusively from monogenetic vents, contain sparse (〈2%) plagioclase phenocrysts that span a remarkably wide and continuous range in composition (≤30 mol % An). Many, but not all, of the plagioclase crystals display diffusion-limited growth textures (e.g. swallow-tails, skeletal, vermiform). Hypotheses to explain the paradox of a wide compositional range despite a low abundance of plagioclase include (1) incorporation of xenocrysts and/or magma mingling, (2) slow crystallization of plagioclase driven by slow cooling in a magma chamber, (3) slow crystallization of plagioclase followed by a resorption (e.g. heating) event, and (4) crystallization driven by rapid degassing (i.e. loss of melt H 2 O) ± rapid cooling during ascent. To test these hypotheses, a series of phase equilibrium experiments were conducted under pure-H 2 O fluid-saturated conditions in a cold-seal pressure vessel between 30 and 300 MPa and 750 and 950°C. The results show that the plagioclase population in each obsidian sample could have grown from their respective melts, with the exception of a single calcic core (An 60–63 ) in one sample. The results additionally rule out slow cooling in a magma chamber, because this would lead to equilibrium abundances of plagioclase (≤20%), which are far higher than what is observed in the samples (〈2%). Nor can resorption (i.e. heating) explain the low abundance of plagioclase, because this would eliminate the more sodic plagioclase crystals and hence the wide compositional range of plagioclase that is observed. The most viable hypothesis is that the sparse plagioclase phenocrysts grew relatively rapidly during magma ascent to the surface; this is consistent with the results of isothermal (850°C) continuous decompression experiments (2·9, 1·0, 0·8, and 0·1 MPa h –1 ), under pure-H 2 O fluid-saturated conditions, which were performed on one of the rhyolites (MLV-36; 73 wt % SiO 2 ) and quenched at P H2O = 89, 58 and 40 MPa. The four decompression rates correspond to degassing rates of 1·6, 0·56, 0·45 and 0·06 wt % H 2 O per day. Decompressions ≥1·0 MPa( P H2O ) h –1 , initiated above the liquidus, quenched to 100% glass at all final P H2O . Decompressions at 0·8 MPa( P H2O ) h –1 , also initiated above the liquidus, led to plagioclase crystals nearly five times larger than those grown in runs decompressed at the same rate, but initiated just below the plagioclase-in curve. It is the kinetic hindrance to nucleation that permits crystal growth to be concentrated on relatively few crystals, leading to larger crystals. Plagioclase growth rates from these experiments show that the largest phenocrysts (~1 mm) in the MLV-36 obsidian could have grown in 〈42 h. A cooling rate of ~1·2°C h –1 closely matches both the increase in melt viscosity with time and the effective undercooling with time that occurs during the 0·8 MPa( P H2O ) h –1 decompression over the first 50 h. The combined results point to crystallization of sparse plagioclase driven by relatively rapid rates of degassing ± cooling during ascent to the surface of melts that were initially above their liquidus. The obsidian samples must have been efficiently segregated as nearly 100% liquids from their respective source regions at H 2 O-fluid undersaturated conditions to attain a degree of superheating upon ascent before reaching fluid saturation.

Description: An updated and expanded data set that consists of 214 plagioclase-liquid equilibrium pairs from 40 experimental studies in the literature is used to recalibrate the thermodynamic model for the plagioclase-liquid hygrometer of Lange et al. (2009) ; the updated model is applicable to metaluminous and alkaline magmas. The model is based on the crystal-liquid exchange reaction between the anorthite (CaAl 2 Si 2 O 8 ) and albite (NaAlSi 3 O 8 ) components, and all available volumetric and calorimetric data for the pure end-member components are used in the revised model. The activities of the crystalline plagioclase components are taken from Holland and Powell (1992) . Of the 214 experiments, 107 are hydrous and 107 are anhydrous. Four criteria were applied for inclusion of experiments in the final data set: (1) crystallinities 〈30%; (2) pure-H 2 O fluid saturated; (3) compositional totals (including H 2 O component) of 97–101% for hydrous quenched glasses and 98.5–101 for anhydrous quenched glasses; and (4) melt viscosities ≤5.2 log 10 Pa·s. The final data set spans a wide range in liquid composition (45–80 wt% SiO 2 ; 1–10 wt% Na 2 O+K 2 O), plagioclase composition (An 17–95 ), temperature (750–1244 °C), pressure (0–350 MPa), and H 2 O content (0–8.3 wt%). The water solubility model of Zhang et al. (2007) was applied to all hydrous experiments. The standard error estimate on the hygrometer model is 0.35 wt% H 2 O, and all liquid compositions are fitted equally well. Application of the model as a thermometer recovers temperatures to within ±12°, on average. Tests of the hygrometer on anhydrous piston-cylinder experiments in the literature, not included in the regression, show that the model is accurate at all pressures where plagioclase is stable. Applications of the hygrometer are made to natural rhyolites (Bishop Tuff, Katmai, and TobaTuff) with reported H 2 O analyses in quartz-hosted melt inclusions from the literature; the results show agreement. Applications of the hygrometer/thermometer are additionally made to natural rhyolites from Iceland and Glass Mountain, California. The updated model can be downloaded either as a program in Excel format or as a MatLab script from the Data Repository.

Description: 40 Ar/ 39 Ar geochronology is used to determine the eruptive history of Volcán Tepetiltic, a predominantly andesitic stratovolcano that underwent caldera collapse during explosive eruption of zoned rhyodacite-rhyolite. The main edifice was largely constructed between 560 and 450 ka, but it was not complete until ca. 416 ka, during which time ~42 km 3 of phenocryst-rich (25–40 vol%) lavas ranging from 57 to 69 wt% SiO 2 were erupted. After a hiatus of ~180 k.y., there was a climactic Plinian eruption of ~4–8 km 3 of zoned magma (68–75 wt% SiO 2 ). Afterward, a small rhyodacite dome (69 wt% SiO 2 ; 190 ± 22 ka) and an andesite dome were emplaced on the caldera floor. There has been no subsequent volcanic activity. The crystal-poor (0–5 vol%) Plinian pumice could not be dated directly, owing to the hydration of glass and the absence of a K-rich mineral phase. Instead, the age of the climactic eruption was bracketed to be ca. 236 ± 52 ka from 40 Ar/ 39 Ar dates (±2) on peripherally erupted basaltic andesite flows, which are found underneath (241 ± 47 ka) and overlying (220 ± 36 ka) the associated fallout deposits. A volume of ~9 km 3 of basaltic andesite was erupted from three peripheral vents surrounding Volcán Tepetiltic, most of it from a shield volcano that was active before, during, and after the Plinian event. Thermal models indicate that the magma chamber beneath Volcán Tepetiltic would have cooled below its solidus within 36 k.y. in the absence of new injections of magma. Given the long hiatus (~180 k.y.) between the cone-building episode that built the andesite stratovolcano and the explosive eruption of rhyodacite-rhyolite, it is proposed that the influx of voluminous basaltic andesite into the upper crust drove partial melting of the subsolidus magma chamber beneath Volcán Tepetiltic, which explains the synchronicity of the basaltic andesite volcanism with the Plinian eruption of zoned, crystal-poor rhyolitic melt.